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In Australia customs and immigration are cleared at the first airport you arrive at (some exceptions for "domestic" international flights). Thus you must clear immigration, collect bags and take through customs and agriculture x-ray at SYD.

As Jetstar does not interline with SQ you will need to take your bags over to their domestic terminal (T2). There is an expensive train or a bus (also a lot for the distance but not as bad as the train). You cannot walk between international and domestic terminals. Since you are not arriving or departing SYD on a QF or oneworld flight, you also cannot take the free QF transfer bus.